1. Field of the Invention
The invention relates to a developing apparatus for developing an electrostatic image formed by the use of an electrophotographic process or an electrostatic recording process, and particularly to a developing apparatus for use in a copying machine, a printer, a facsimile apparatus or the like.
2. Description of the Related Art
In conventional image forming apparatuses, and above all, image forming apparatuses of an electrophotographic type for effecting color image forming, there has been widely utilized a two-component developing process using a mixture of a nonmagnetic toner and a magnetic carrier as a developer.
The two-component developing process, as compared with presently proposed other developing processes, has such merits as the stability of image and the durability of the apparatus, while on the other hand, it has unavoidably suffered from the deterioration of the developer, particularly the deterioration of the carrier, due to long-period endurance and therefore, it is necessary to do the work of interchanging the developer with the long-period use of the image forming apparatus.
On the other hand, in the recent full-color copying machines/printers, for the curtailment of running costs, the introduction of the following technique has been progressed as a method of maintaining a high quality of image without doing the work of interchanging the developer.
For example, according to Japanese Patent Publication (Koukoku) No. 2-21591, “in a developing apparatus for an electrophotographic copying machine provided with agitating means for agitating a carrier and a toner, and a developing roll for supplying a developer agitated by the agitating means to a photosensitive member, a carrier supplying device and a toner supplying device are separating or integrally provided above the agitating means, and a developer overflowing portion is provided in the side wall of the housing of a developing apparatus and therefore, a fresh developer can be supplied little by little by the supplying devices and also can be discharged from the developer overflowing portion, and the characteristic of the developer in the housing of the developing apparatus can be maintained constant with a result that the quality of the image of a copying can also be kept constant,” and “the oldened developer in the housing of the developing apparatus is sequentially automatically discharged from the developer overflowing portion and therefore, the cumbersome developer interchanging work of removing the developing apparatus from the copying machine, taking out the old developer in the housing of the developing apparatus and refilling the housing with a fresh developer, and thereafter mounting the developing apparatus again becomes unnecessary and moreover, the scattering of the developer is prevented and this is hygienic”.
That is, the deteriorated developer (carrier) is gradually changed with fresh material, whereby the progression of the apparent deterioration of the carrier is stopped and as to the entire developer, the characteristic thereof is stabilized. Thereby, the work of interchanging the developer is made unnecessary and a maintenance property is improved.
In the recent full-color copying machines/printers, however, in order to achieve a high quality of image, the smaller particle diameters of the toner and carrier in the developer have been progressed, but when the toner and carrier are made small in particle diameter, a change in the triboelectricity of the developer for a change in the amount of moisture in the atmosphere becomes great. Along therewith, a change in the bulk density of the developer in the developing apparatus also becomes great. Specifically, when the amount of moisture becomes small, the triboelectricity becomes high and the bulk density becomes low. Conversely, when the amount of moisture becomes great, the triboelectricity becomes low and the bulk density becomes high.
When in this state, the amount of developer is made constant, the bulk density becomes low and the level of the developer rises, or the bulk density becomes high and the level of the developer falls.
In the conventional art, as described above, the method adopted is a method of providing a developer overflowing portion, i.e., a developer discharge port, in the side wall of the developing apparatus, effecting the discharge of the developer by overflow, and maintaining the level of the developer constant.
However, when as described above, the level of the developer is not stable due to a change in the bulk density of the developer, the discharge of the developer is not effected suitably, and the balance between the amount of supplied developer and the amount of discharged developer becomes bad and the amount of developer in the developing apparatus is gradually increased and the level of the developer rises.
Consequently, when the level of the developer at the other locations than the developer discharge port becomes too high when the developer is discharged through the developer discharging port, if the amount of developer is too much increased due to the faulty agitation of the supplied toner or the above-described change in the bulk density of the developer, there has been a case when overtorque occurs and such a problem that a high quality of image cannot be maintained arises.